It is known that vinylidene fluoride (VDF) can be polymerized in aqueous emulsion using inorganic or organic peroxides as initiators. Among the latter, diterbutylperoxide (DTBP), as described in U.S. Pat. No. 3,193,539, has resulted particularly effective. With such initiator the polymerization is carried out at high temperatures, generally comprised between 120.degree. and 130.degree. C., and at a pressure comprised between 300 and 1000 psig, without employing any chain transfer agent. The PVDF obtained with such process shows a low cristallinity degree, with a high amount of structural defects (tail-to-tail and head-to-head inversions, branchings, microgels), and consequently unsatisfactory mechanical properties and mouldability, quite unsuitable for the manufacture of articles, such as pipes, valves, fittings, tanks, heat exchangers, membranes, flanges, etc, to be employed in the chemical industry.
In U.S. Pat. No. 3,475,396 a VDF polymerization process is described, wherein diisopropyl peroxydicarbonate (IPP) is used as initiator, optionally in the presence of a suitable chain transfer agent. IPP is characterized by a half-life of 1 hour at 61.degree. C., therefore the process can be carried out at lower temperatures in comparison to DTBP, generally comprised between 65.degree. and 85.degree. C., with reaction times lower than 6 hours. The low polymerization temperature allows to obtain a PVDF having high mechanical properties, since the structural defects are drastically reduced and therefore the crystallinity of the product is higher.
The PVDF obtained with the low temperature process mentioned above, though having high crystallinity degree and very good mechanical characteristics, shows poor thermochemical resistance to dehydrofluorination. In fact, on the basis of the experimentation carried out by the Applicant, it has been found that the PVDF obtained at low temperature, if submitted to high temperatures (over 100.degree. C.) or contacted with basic solutions (for instance NaOH at 10%), tends to dehydrofluorinating, with formation in the chain of conjugated double bonds of the --CF.dbd.CH--CF.dbd.CH-- type. The formation of such structural defects, besides conferring undesired colorations, lowers considerably the mechanical resistance of the material, as shown by measurements of chemical stress cracking.
Moreover, the low temperature process shows several drawbacks especially connected to the use of extremely hazardous peroxidic initiators, such as IPP, since explosive also at low temperatures. For instance, IPP has a self-accelerating decomposition temperature (SADT) of about 10.degree. C. and requires a shipment temperature lower than -10.degree. C. This requires to take particular safety measures both for transportation and storage, and during its use in the polymerization process. Because of the hazard with such products, in some countries their use on an industrial scale was submitted to severe restrictions.